Problem: The scale on a map is 6cm : 2km. If the distance between two cities is 10km, how far apart in cm are the two cities on the map?
Answer: The scale means that every 6cm on the map corresponds to an actual distance of 2km. An actual distance of 10km is the same as 5 $\cdot$ 2km. The distance between the two cities on the map is 5 $\cdot$ 6cm, or 30cm.